Inactivated SIV or recombinant subunit vaccines have provided limited or no protection against infection with SIV. Macaques vaccinated with live sivmac239 with a deletion in the nef gene (SIV FNEF) resisted challenge with virulent SIV. However, SIV fnef persists indefinitely in macaques, provides limited or no protection until a year or more after immunization, and is pathogenic to neonates; these drawbacks limit its use as a vaccine. We have previously demonstrated the attenuating and immune enhancing activities of interferon-gamma (IFN-G) in viral infections. We hypothesized that a vaccine of SIV FNEF expressing IFN-G would have enhanced safety and efficacy for macaques. We have generated SIV FNEF recombinants that contain the human IFN-G gene under the control of the SV40 early promoter. SIVSV-IFN contains the SV40-IFN-G cassette in the sense orientation, whereas for SIVNFI-VS this insertion is in the anti-sense orientation. Only SIVSV-IFN expresses moderate levels of IFN-G ; cells infected with SIVNFI-vs manifested minimum activity (20 units/ml) after 11 days in culture. However, both recombinant viruses are unstable. Rhesus macaques were given 2000 TCID50 of these viruses; cell-associated virus loads were determined in PBMCS and lymph nodes (LN). Animals were challenged, along with four SIV FNEF-infected and two naive macaques, with 100 AID50 of SIVMAC251. Post-challenge studies are in progress.